The aim is to develop the guinea pig as an animal model system for the effects of prenatal administration of estrogen (diethylstilbestrol or estradiol) to the pregnant mothers on male sexual development. This is in response to reports of effects in the male offspring of women exposed during pregnancy to estrogens, chiefly diethylstilbestrol. Critical periods for sexual differentiation and cytodifferentiation will be defined by serial sections of embryos and fetuses throughout pregnancy. The effects of prenatal exposure to estrogen will be studied in relation to the histology and fine structure, where appropriate, of the gonads and accessory organs. The effect of estrogen exposure on the subsequent responses of the sexual structures to endogenous sex hormones before birth, a normal feature of the guinea pig as well as the human baby, will be studied. Effects on postnatal, prepuberal, and adult structure will also be determined. Sexual function will be assessed by histological study of gonads, epididymis, and accessory organs of reproduction, by studies on the fertilizing ability of the spermatozoa, mating responses, and on the ability to promote successful pregnancies in fertile females. Late effects of prenatal estrogen on the aging process in the reproductive organs, such as metaplasia or neoplasia, will also be studied.